1. Field of the Invention
The present invention relates to a method and an apparatus for severing a printed material web in a folding apparatus of a web-fed rotary printing press.
2. State of the Art
In conventional folding apparatuses as described, for example, in U.S. Pat. No. 5,334,129, rotating cutting cylinder systems having one or more blades mounted to the periphery of the cutting cylinder and extending across the width of the running paper web are used for cross-cutting the web. The web is cut into signatures as the cutting cylinder is rotated about an axis and the cutters are brought into contact with the travelling web. Once the web is cut into signatures, the signatures are fed to a cross folding unit for further processing.
However, every time the web is severed in typical cross-cutting cylinder systems, large impulses are imparted through the drive system of the printing press causing periodic web tension variations. These variations in tension are transmitted back through the web with each cut and thus adversely affect the quality of the printed images. Furthermore, when the knife or blade of known cross cutting cylinder systems swipes past the paper, this can cause the paper to follow the knife resulting in print-to-cut register errors and therefore reducing the ability for these systems to perform finish quality cuts.
Also, when processing single-layer or multi-layer webs, known prior art cutting systems show another deficiency in that the signatures are not completely restrained during and after the cutting operation. As a result, the leading end of the multi-layered web, which is created with each cut, is likely to open up and cause a jam in or damage to the folding section.
Furthermore, due to the high working load, the blades of the known prior art cross-cutting cylinders become dull after a comparatively short time and have to be replaced or resharpened frequently, in order to maintain a sufficient quality of the cut edges. This, in turn, reduces efficiency because the printer must be stopped to replace or resharpen the blades.